Judge a pack on its entire content, that is to say even the things that remain persistent throughout your game after you finished the related missions.
I've not included the armor/outfits DLC's.
Myself, I'm partial to Overlord. I thought it was a very interesting storyline that finally managed to do justice to the hovertank as a gameplay element and appealed to me philosophically aswell. I enjoyed the final section where you experience the entire world in a sort of binary code, and I thought the combat sections throughout the missions were very strong. The leveldesign was amazing too, that Geth ship was a very eerie place.
Second for me comes Kasumi's DLC, I loved the entire heist take on a space opera. Third comes the Shadow Broker DLC which for some reason just failed to really captivate me. I don't know why. I thought the Shadow Broker himself was a letdown, storywise. I also think he shouldn't have been uncovered in a DLC. And I didn't really like the leveldesign as much as I did in Overlord and Kasumi's mission. The car chase section was amazing fun though, but that's pretty much the only highlight.